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First Atomic Laser

Physicists have created a new laser in which a beam of atoms march in lockstep, like the photons of a light laser. The remarkable achievement, reported in the 31 January issue of Science, could eventually lead to better methods of etching tiny electric circuits and to improvements in superaccurate atomic clocks.

This interference pattern of two overlapping Bose-Einstein condensates shows that they are "laserlike." The density of atomic sodium (vertical axis, in false color) is shown in an area of 0.1 millimeter by 0.4 millimeter containing about 50,000 atoms.

Two years ago, physicists achieved the crucial starting point for an atom laser when they created an exotic state of matter known as a Bose-Einstein condensate, a dense cloud of atoms cooled in a magnetic trap to within an iota of absolute zero. Now, a group at the Massachusetts Institute of Technology led by Wolfgang Ketterle has shaped this novel material into pulses of atoms that have the hallmarks of a laser beam.

The first step, creating what laser physicists call an output coupler to extract sodium atoms from a trap, was relatively easy. The trap, Ketterle says, "can be described loosely as like atoms bouncing back and forth between magnetic walls." The walls, however, only retain atoms whose spin axis is pointing up. Flip those spins, Ketterle says, and "the restoring forces become expelling or repulsive forces." So his team simply applied another magnetic field to the atoms, which tilted their spins to any desired angle. By controlling the angle, the researchers could then "pulse out" portions of the condensate. The details of this step are being published today in Physical Review Letters.

The tough part was showing that all the atoms in these dollops of condensate are coherent--in the parlance of quantum mechanics, that their wave functions are oscillating up and down in phase, just as laser light waves are in phase. First, Ketterle and his colleagues created two condensates by beaming a laser up through the middle of their magnetic trap. The laser light repelled the atoms and split the condensate into two distinct halves. For this test, the group simply turned off the trap and let the condensates "free fall"--expand into the surrounding vacuum. The condensates swelled until they overlapped and interfered like light waves, demonstrating the atomic version of the bright and dark fringes in an interference pattern.

"The experiments are gorgeous," says Oxford University physicist Keith Burnett, and the demonstration that this is really a laser is "the most beautiful clear evidence."

The technology does seem to come with a handicap: Unlike light, an atomic laser beam can't propagate freely through the atmosphere. Nevertheless, any field that relies on beams of atoms might benefit from the brighter and better controlled beams. Atomic clocks, which are based on the vibrations of atoms drifting through a cavity, are one candidate. Another is nanolithography, the technique by which circuit designers lay out minuscule features. It now depends on a mask or stencil to control where atoms or light land on a surface, but an atom laser--which could be focused and directed like a light laser--might provide a way of writing the patterns directly.